But a wheelchair user shouldn't have to be guilt-tripped into letting someone else stay in the space! Surely that's just embarrassing for everyone concerned, to start negotiating who needs the space the most. Just because some disabled people would be gracious enough to suggest you should take their space, does not mean that you should act as though this is what should happen.
People who use buses with buggies (of which I am one, daily), need to remember that every time they do so with a buggy that it is "impossible" to fold and bags of shopping that are too difficult to "juggle" (why do you need to juggle them - just put them on the floor 
), they are taking a gamble that the space won't be needed by a wheelchair user. And, in my experience, the space isn't usually needed.
But sometimes, very occasionally, it is needed.
And then you have to get out of the way.
And not being able to manage your shopping or fold your buggy is not an excuse.
You knew when you got on that this could happen. So if you are taking a journey where you really really have to stay on the bus and can't afford to be made to get off, you need to work out ways of dealing with it.
You could:
If you know you are going shopping, take a backpack and put everything in there, then you won't have to juggle shopping bags.
Take a sling with you for the baby and reins for your older child. Or have your tiny baby/babies in a soft carrycot with handles that you can easily remove from the buggy. Fold your buggy up before you even get on the bus.
If you often have to take the bus, choose a small pushchair that can be folded/reopened with one hand.
Yes, it is less convenient. But it is perfectly possible.
Disabled people spend their whole lives having to think creatively and come up with ways of accessing things in daily life that everyone else takes for granted.